Specific antisera to myelin basic protein (BP) and its fragments have been used to study (A) the relationship of antibody-binding to conformation; (B) location of specific antigenic and encephalitogenic regions within the BP molecule; and (C) cytochemical localization of BP in developmental studies. Rabbit antisera to mammalian central nervous system (CNS) BP have been analyzed for their binding specificity to 125I-labelled fragments of BP. From our data, we conclude that rabbit lymphocytes recognize and produce antibody to an amino acid sequence in the N-terminal region between residues 1 and 36; at least two peptide sequences in the next 52 residues (37-88); and one or more sequences in the last 80 residues (89-169). Individual animals may recognize different combinations of regions but no one site stands out as being a major binding site as was the case with guinea pig anti-BP sera.